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multimeter protocol

Discussion in 'Electronic Design' started by [email protected], Aug 18, 2008.

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  1. Guest

    Dear All,

    I have a Mastech MS8220T multimeter that I want to hook up to a PC.
    It has a serial interface and the included program runs fine, but the
    protocol is not documented. I had a look at the output with a scope,
    looks like 9600 baud, but there are some bits that are too short for
    that, they have a bit less than 50 us duration. Maybe these are the
    stopbits, signal quality is very bad and it's difficult to identify
    the individual bits.
    Do you have any Idea how to guess the protocol and rs232 settings?
    Is there any way to identify the port settings if I have the software
    running (xp) ?
    I had a look at the standard 7 segment encoding, but this does not
    seem to be used. Even if the disply shows a constant value several
    different codes are transmitted.

    Any ideas?

    Dirk - Transcendence UK - A UK political party - Our podcasts on weird stuff
  3. Guest

    Hi Dirk,

    thanks a lot for your link, the device monitoring studio did it in
    less than five minutes.
    Here are the settings for the Mastech, in case anyone should have the
    same problem:
    Baudrate: 19200
    7 bit
    1 stop
    Odd parity

    I never would have imagined this speed.
    The data is transmitted as plain ASCII and has the form

    m is the multiplier 0 for mV, 1 times 10, 2 times 100
    n are the four digits
    x give sign, range etc
    for DCV it is ;80: for pos
    ;<0: for neg
    m n1 n2 n3 n4 x1 x2 x3 x4

    if you write x in binary, bit 19 is the sign bit.

    Thanks for your help!

  4. Guest

    Hi Donald,

    when I hooked up the scope the additional loading was already too much
    to stop the thing from working, so probably it
    wouldn't work with an extra port either, therefore I didn't even try.
    This is the first DMM I've seen that runs at such a high speed, I
    thought the 50 us were glitches as well, because the signal looks very
    bad, already the 104 us (now I know two bits...) are barely defined.

  5. PeterD

    PeterD Guest

    Either you really screwed up your connecting the scope, or this is not

    Do as Donald suggests, that is a valid, and easy thing to do. If it
    doesn't work because the second monitoring port loads the device too
    much, then it is not RS-232.
  6. Guest

    probably it is not true rs-232, the DMM contains a photoemitter and
    the plug a photodiode and three transistors.
    Does not matter, works good enough now. With the right port settings
    it was easy to guess.


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